This paper deals with reaching and keeping the formation for a group of mobile robots. A set of virtual targets (points) form a virtual structure of the same shape as the desired formation. Hence, to join and to keep this formation, each robot has only to track one of these targets. The objective of the paper is mainly to propose a cooperative strategy between the robots in order to rapidly join the virtual structure. Instead of assigning ahead one target per robot, the proposed strategy consists of making each one able to negotiate the closest target. If the latter is desired by more than one robot, it is left for the one which meets more difficulties to find an other target. Negotiation is based on a minimalist communication of relative cost coefficients between the robots. Simulation and experimental results validate the proposed contributions.